A geo-fence is a virtual area defined by co-ordinates of a geographic mapping system the shape of the geo-fence is not limited by terrain, buildings or other boundaries physical or political.
Determination of a geographic location (geo-location) using the same mapping system that is used to define the geo-fence can be achieved using a multitude of mechanisms. By way of example, one such mechanism is the Global Positioning System (GPS). GPS includes multiple satellites and a sophisticated array of ground stations and controls to broadcast coded radio frequencies that when received and decoded by a purpose built electronic device can output time and location within the GPS geographic mapping system. GPS is but one of many such geo-locating systems.
It is possible, having decided on a geo-location system and the associated geographic framework to adapt the geo-location determined by one system to another but this is not typical.
The word geo-location can also be determined by latitude and longitude coordinates of a particular location and may also include altitude. Terms and definitions like latitude, longitude and altitude are standardized by ISO/IEC 19762-5:2008.
There will first be a predetermined location (as defined by a determined latitude and longitude pair) such as at a shop, place of business, toll-way entrance, event entrance, etc. and in the simplest version of a geo-fence, a single radial distance is used to define a circular boundary of the geo-fence. Thus all the latitudes and longitude pairs that exist within the circular area defined by the geo-fence are known. Entry to or proximity to such a virtual fence can then be determined by matching the determined latitude and longitude pair of the device which determines its location with all the pairs within the geo-fenced area, and if there is a match the device is within the geo-fence.
The detection of entry into a geo-fenced area can be used to alert the user of the mobile device that they are at that time likely to be close to the shop, place of business, entered the toll-way, or entered the event, etc. This information is potentially valuable to the user and particularly valuable to the shop, business, toll-way and event organizer in numerous ways some of which will be helpful and most of which will be or economic worth to the user or business.
There is much more to the process than simply described above but the principle should be apparent.
The shape of the fence is typically determined by the intended use of the geo-fence and the related function or result of detection of entry to the geo-fenced area.
Geo-fencing as a technique is used to understand if a device has entered any previously determined geographic location. The prerequisites for a device to locate itself are as follows:    A device with the capability to use one of many methods to determine its location.    A device with the ability to maintain a log of predetermined areas/lines/points, expressed as a combination of coordinates as defined by a standardized geodetic system or framework—i.e. WGS84 ellipsoid.    A device with the capability to calculate and ascertain if a location that has been established for the device, is located within or without a predetermined area and communicate this awareness or perform actions based on it.
Most modern computer devices and particularly those that have mobile telephonic capabilities will have a location determination capability which is used by one or more of the applications on the mobile device sporadically, at defined, or undefined intervals. FIG. 1 illustrates the prior process of mobile device determination of entry of the mobile device into a predetermined geo-fenced area.
FIG. 1 depicts a prior art approach to geo-fencing where a predetermined location is identified as a latitude/longitude (herein referred to as a lat/long pair for brevity) is depicted as lat/long pair (lt0, lg0) for example, 34.9290° S, 138.6010° E, which for the purposes of the example, is a city council office location. The calculation of the lat/long envelope of a circle about the lat/long pair having a radius ‘r’ is non-trivial since it is necessary to accommodate the WGS84 ellipsoid sphere assumed to apply to the curvature of the earth but for relatively short distances (say less than 1 kilometer then a flat earth approximation will suffice) and in this case 100 meters is the chosen radius r. However, there is more than one software program that will provide the information required to allow for a determination as to whether a particular lat/long pair determined by a mobile device lies within the envelope of the radius of the origin predetermined location lat/long pair of (lt0, lg0).
As indicted in FIG. 1, the first time a geolocation of the lat/long of the mobile device is determined and the lat/long pair is (lt1,lg1) and the second time a geolocation of the lat/long of the mobile device and the lat/long pair is (lg2,lg2).
A comparison of the first and second lat/long pairs with any of the lat/long pairs within the 100 meter radius would not find any coincidence even though the mobile device passed within that radius.